Radio frequency heating apparatus



Sep; 26, 1967 J. G. D. MANWARING 3,344,254

RADIO FRQUENCY HEATING APPARATUS Filed Aug. e, 196e Y 2 sheets-sheet 1 IFIG. 5

United States Patent O 3,344,254 RADIO FREQUENCY HEATING APPARATUS Joshua George Dowell Manwaring, Needham, Mass., as-

signor to Radio Frequency Company Inc., Medtield, Mass., a corporation of Massachusetts Filed Aug. 8, 1966, Ser. No. 570,786 9 Claims. '(Cl. 219-10.81)

ABSTRACT OF THE DISCLOSURE Radio frequency heating apparatus including a conductive, sheet metal, open-ended enclosure of generally rectangular cross-section having within it a lumped constant .resonant tank circuit element. Such element consists of a plurality of electrically conductive plates generally parallel to one another and perpendicular to and along a wall of the enclosure, said plates `being spaced from one another ina direction perpendicular to the plane of said plates for capacitive coupling therebetween to provide pairs of mutually inductively coupled inductors, and forming tw-o `series of plates arranged alternately with one another. A radio frequency generator is connected to one of the series of plates for generating a radio frequency field. A stray field electrode structure within the en- -closure is electrically coupled to the enclosure for heating material positioned adjacent to it. It includes two pairs of elongated electrically conductive plate electrodes in substantially co-planar, spaced-apart relation mounted substantially parallel to one of said wall members of said enclosure and perpendicular to said plates and a plurality of elongated, substantially V-cross section electrode elements positioned in opposed parallel, spaced-apart relation generally parallel to and spaced apart from said plate electrodes on the side thereof opposite the wall of the enclosure. The electrode elements form two series of electrode elements alternately arranged with one another with the longitudinal axis of each of said plate electrodes substantially perpendicular to the longitudinal axis of each of said electrode elements. The plate electrodes are positioned intermediate said wall and said electrode elements generally .are parallel to and spaced apart from the electrode elements and the wall. One of the series of said electrode elements is electrically connected to each pair of said plate electrodes.

This invention relates to radio frequency circuitry and components and, more particularly, to radio frequency heating equipment ,and electrode systems useful therein.

It is a primary object of the present invention to provide a compact, relatively simple, easily cleaned stray field electrode system which eliminates the heretofo-re serious problem of -arcing between electrodes without decreasing the strength of the radio frequency heating field. Other objects include providing an electrode system which is of simple and rugged construction and which may be readily tuned to accommodate a variety of conditions.

The invention accomplishes these and other objects by providing a stray field electrode system comprising a pair of co-planar conductive plates and a plurality of elongated, Vorosssection electrode elements positioned in parallel, spacedapart relation -generally parallel to and spaced apart from the conductive plates. The electrode elements are alternately arranged to form two series of electrodes, one of which is electrically connected to each vof the conductive plates.

The electrode system provided by the present invention is particularly useful in combination with radio frequency heating apparatus utilizing a tuned load circuit comprising a generally tubular electrically conductive enclosure, preferably of rectangular cross-section with a pair of opposite ends spaced a substantial distance from one another, with a plate circuit component having its series of plates mounted within the enclosure perpendicular to its walls and extending therealong throughout the length of the enclosure between its open ends. The plate circuit is connected to the enclosure and also to a radio frequency generator for heating material positioned Within or Without the enclosure.

Other objects, features and advantages will appear from the following detailed description of a preferred embodiment of the invention, taken together with the attached drawings in which:

FIG. 1 is a perspective view of a radio frequency heating apparatus including a preferred embodiment of the present invention;

FIG. 2 is a perspective view of a stray lield electrode system embodying the present invention and used as a component in the apparatus of FIG. l; and

FIG. 3 is a sectional end view of the electrode system of FIG. 2.

Referring more particularly to the drawings, the radio frequency heating apparatus there illustrated comprises a generally tubular, open-ended conductive loop enclosure, designated 10, preferably of highly conductive sheet metal such as aluminum or copper and of rectangular crossesection. Within enclosure 10 and extending along one side thereof are a plurality of slotted plates providing a lumped capacitance and inductive resonant tank circuit, generally designated 12, of the type disclosed in my Patent No. 3,095,548, inductively and capacitively coupled to said enclosure. A novel and improved electrode system, generally designated 14, of the `so-called stray field type is provided in the substantial transverse space within conductive loop enclosure 10 between tank circuit 12 and the opposite wall of the enclosure and is capacitively coupled to tank circuit 12. A conveyor belt 16 trained around pulleys 17 and 18 mounted adjacent opposite axial ends of enclosure 18 is provided for conveying material to be treated through the conductive loop enclosure.

More specifically, enclosure 10 includes two pairs of o-pposite, spaced parallel, electrically connected, sheet metal wall members, upper and lower wall members 20 and 22 respectively, and side Wall members 24 'and 26 respectively, providing :an axially extended, electrically conductive loop having a pair of opposite open ends spaced a substantial distance from one `another along the axis of the enclosure.

The lumped constant tank circuit 12 provided by the plates Within enclosure 10 consists of a plurality of electrically conductive plates generally parallel to one another and perpendicular to said wall members 20, 22, 24 and 26. The plates, which form two alternating series of plates 28 and 30 having therein, respectively, transverse, alternately extending slots 29 and 31 with enlarged circular inner ends coaxial with one another, are spaced from one another in a direction perpendicular t-o the plane of said plates for capacitive coupling therebetween. The plates and slots are arranged alternately with one another and are electrically connected at a common point 32 to define a radio frequency current path in one series of yplates 28 of opposite direction from that in the other be referred to for convenience as a high energy point, on

one plate only of one of said series of plates. Power source 34 thereby provides radio frequency power to tank circuit 12 and to conductive loop enclosure 10.

From an electrical standpoint, conductive loop enclosure preferably has a peripheral dimension of about a full wave-length of the frequency of power source 34, although this has been found not to be critical. The axial length of enclosure 10, even up to lengths greater than one wavelength, does not deleteriously aect the operation of the apparatus, which makes possible the simultaneous heating of an extended segment of material while being conveyed through the apparatus on conveyor belt 16. The extended length of enclosure 10 makes possible the treatment for a substantial interval of material being conveyed through the apparatus at a substantial rate, so that the apparatus is particularly useful, for example, in the drying of sheet or particulate material as it is being conveyed therethrough.

As shown in FIGS. 2 and 3, stray field electrode system 14, which is capacitively coupled to enclosure 10 and tank circuit 12, comprises two pairs of substantially coplanar, electrically conductive plate electrodes, pair 40 and pair 42, arranged alternately and spaced apart from each other with their planes substantially parallel with and spaced part from upper wall member of enclosure 10. Each of said plate electrodes is secured at its opposite longitudinal ends, by means of insulators 44 and 46, to an adjustable plate 48 interposed intermediate said plate electrodes and said upper wall member. Plate 48 is mounted on wall member 20 for vertical adjustment relative thereto by a plurality of insulated rotatable mounting members 50 having operating handles 52 and screw threads cooper-ating with suitable threaded bores in said upper wall member.

Pairs 40 and 42 of said plate electrodes are electrically connected by a lplurality of tunable inductive hairpin loops 54 of conductive sheet metal mounted within the cavity ,between said plate electrodes and plate 44 at lsubstantially right angles to the longitudinal axis of said plate electrodes. A pair of shorting members 56 may be slidably mounted on each of hairpin loops 54 for tuning, if desired.

For producing the stray field, a plurality of elongated, V-cross-section pointed end, transversely extending, stray field electrode elements are mounted on each of said pairs of said plate electrodes on the side thereof opposite plate 44. Each of said stray field electrode elements is secured to one of said pairs of said plate electrodes in position with said electrode element substantially parallel to the upper and lower walls of enclosure 10 .and spaced from it with its longitudinal axis substantially parallel and at right angles to said pairs of plate electrodes with their pointed ends pointing away from the plate electrodes as shown. The stray field electrode elements are alternately arranged to form two series, 58 and 60x Each electrode of one of said series, series 58, is secured to and thereby electrically connected to a pair 40 of said plate electrodes. Each electrode of the other of said series, series 60, is secured and electrically connected to pair 42 of said plate electrodes.

In the illustrated embodiment, the electrode elements are mounted in substantially a common plane for surface heating on one side of the material passing adjacent thereto. For heating through material or heating both surfaces thereof, it is preferable that the two series of electrode elements be arranged in spaced planes for passage of the material to be heated therebetween, with the electrode elements of each series opposite to or staggered with respect to one another.

The illustrated radio frequency heating apparatus is operated simply by energizing it by means of power source 34 at a frequency of, say 10 to 100 megacycles, and operating conveyor belt 16 to carry the material to be treated through the radio frequency energy field established within enclosure 10. As so operated, the transversely elongated stray field electrode elements, with each series of said elements electrically coupled to enclosure 10 by its respective pair of plate electrodes, provides a radio frequency heating field which extends between adjacent stray field electrode elements throughout the series thereof.

As previously mentioned, arcing between the adjacent electrode elements has heretofore been a serious problem in many radio frequency heating devices, causing a large power loss and a corresponding decrease in the eiciency and effectiveness of the heating device. It is well-known that arcing can be substantially eliminated by increasing the distance between the electrode elements, but increasing this distance has the undesired effect of decreasing the intensity of the radio frequency heating field. The `stray field electrode system provided by the present invention effectively eliminates arcing by increasing the eyfectz've arcing distance between adjacent electrodes without increasing the actual distance between the same two electrodes. As shown in FIG. 3, each of the stray field electrode elements is mounted with the point of its V-cross-section directed away from the plate electrodes to which the electrode element is secured. The distance between the V-points of adjacent electrodes is significantly greater than the distance between the most closely adjacent points ofthe electrodes, and, any electric arcs will tend to move toward and occur at the V-points of the electrode elements, the effective arcing distance between adjacent electrodes is thereby greatly increased.

Other embodiments will occur to those skilled in the art and are within the following claims.

What is claimed is:

1. A stray field electrode comprising, in combination:

first and second substantially co-planar, spaced apart electrically conductive plate electrodes; and,

a plurality of elongated, substantially V-cross-section,

pointed end, electrode elements positioned in parallel, spaced-apart relation `generally parallel to and spaced apart from said plate electrodes with their pointed ends pointing away from said plate electrodes,

said electrode elements forming two series lof electrode elements alternately arranged with one another, one of said series being electrically connected to said first plate electrode and the other of said series being electrically connected to said second plate electrode.

2. A stray field electrode comprising7 in combination two pairs of elongated electrically conductive plate electrodes positioned in substantially co-planar, spaced-apart relation, and

a plurality of elongated, substantially V-cross-section electrode elements positioned in opposed, parallel, spaced apart relation generally parallel to and spaced apart from said plate electrodes,

said electrode elements forming two series of electrode elements alternately arranged with one another with the longitudinal axis of each of said plate electrodes substantially perpendicular to the longitudinal axis of each of said electrode elements, one of said series of electrode elements being electrically connected to each pair of said plate electrodes.

3. The stray field electrode of claim 2 including a plurality of hair-pin loops electrically connected to said plate electrodes, at least `one shorting member being slidably mounted on each of said hairpin loops.

4. The stray field electrode of claim 2 including two pairs `of elongated plate electrodes positioned in co-planar, spaced-apart relationship generally parallel to and spaced apart from said electrode elements with the longitudinal axis of each of said plate electrodes substantially perpendicular to the longitudinal axis of each of said electrode elements, one :of said series of electrode elements being connected to each pair of said plate electrodes,

insulating members .secured to the end portions of said plate electrodes on the side thereof opposite said electrode elements,

a plate member secured to said insulating members in position substantially parallel to and spaced apart from said plate electrodes, and

a plurality of hairpin loops electrically connected to said plate electrodes positioned intermediate said plate electrodes and said plate member, at least one shorting member being slidably mounted on each of said hairpin loops.

S. In combination with radio frequency apparatus including a conductive enclosure of generally rectangular crosssection having at least two pairs of opposite, spaced, parallel, connected sheet wall members,

plurality of electrically conductive plates generally parallel to one another and perpendicular to said Wall members, said plates being spaced from one another in a direction perpendicular to the plane of said plates for capacitive coupling therebetween to provide pairs of mutually inductively coupled inductors, and forming two series of plates arranged alternately with one another to define a radio frequency current path in one series of plates of opposite direction from that in the other series of plates,

mounting means for mounting said plates within said enclosure and generally perpendicular to and along a wall of said enclosure, and

radio frequency generator means connected to at least one of said series of plates for generating a radio frequency field;

stray field electrode means electrically coupled to said enclosure for heating material positioned adjacent said stray field electrode means by said radio frequency field, said stray field electrode means comprising tirst and second substantially co-planar, spaced-apart electrically conductive plate electrodes mounted sub- .stantially parallel to one yof said Wall members of said enclosure and perpendicular to said plates, and

a plurality of elongated, substantially Vcross-section,

pointed end, electrode elements positioned in parallel, spaced-apart relation generally parallel to and spaced apart from said plate electrodes with their pointed ends pointing away from said plate electrodes on the side thereof opposite -said one wall member, said electrode elements forming two series of electrode elements alternately arranged with one another, Ione of said series being electrically connected to said iirst plate electrode and the other of said series being electrically connected to said second plate electrode.

6. In combination with radio frequency apparatus including a conductive enclosure of generally rectangular crosssection having at least two pairs of opposite, spaced, parallel, connected sheet wall members7 a plurality of electrically conductive plates generally parallel to one another and perpendicular to said wall members, said plates being spaced `from one another in a direction perpendicular to the plane of said plates for capacitive coupling therebetween to provide pairs `of mutually inductively coupled inductors, and forming two series of plates arranged alternately With one another to dene a radio frequency current path in one series of plates of opposite direction from that in the other series of plates,

mounting means for mounting said plates within said enclosure and generally perpendicular to and along a wall of said enclosure,

radio frequency generator means connected to at least one of said series of plates for generating a radio frequency field, and

stray field electrode means electrically coupled to said enclosure for heating material positioned adjacent said stray field electrode means by said radio frequency field, said stray field electrode means comprising two pairs of elongated electrically conductive plate electrodes in substantially co-planar, spaced-apart relation mounted substantially parallel to one of said wall members of said enclosure and perpendicular t0 said plates, and

a plurality of elongated substantially V-cross section electrode elements positioned in opposed, parallel, spaced-apart relation generally parallel to and spaced apart from said plate electrodes on the side thereof opposite said one wall member,

said electrode elements forming two series of electrode elements alternately arranged with one another with the longitudinal axis of each of said plate electrodes substantially perpendicular to the longitudinal axis of each of said electrode elements,

said plate electrode being positioned intermediate said i one wall member and said electrode elements generally parallel to and spaced apart from said electrode elements and said one wall member,

one of said series of said electrode elements being electrically connected to each pair of said plate electrodes.

7. Radio frequency apparatus as claimed in claim 6 including a plurality of hairpin loops positioned intermediate said plate electrodes and said one `wall member and electrically connected to said plate electrodes, and at least one shorting member slidably mounted on each of said hairpin loops.

8. Radio frequency apparatus as claimed in claim 6 including two pairs of elongated plate electrodes positioned intermediate said one Wall member and said electrode elements in substantially coeplanar, spaced-apart relation generally parallel to and spaced apart from said electrode elements and said one wall member With the longitudinal axis of each of said electrode elements perpendicular to the longitudinal axis of each of said plate electrodes, one of said series of electrode elements being connected to each pair of said plate electrodes,

insulating members secured to the end portions of said plate electrodes on the side thereof adjacent said one wall member,

a plate member secured to said insulating members in position intermediate said plate electrodes and said one wall member and substantially parallel to said plate electrodes, and

means for mounting said plate member on said one wall member for adjustment relative thereto.

9. Radio frequency apparatus as claimed in claim 8 including a plurality of hairpin loops electrically connected to said plate electrodes positioned intermediate said plate electrodes and said plate member and at least one shorting member slidably mounted on each of said hairpin loops.

References Cited UNITED STATES PATENTS 2,226,871 12/1940 Nicholas 2l9-10.81 X 2,319,174 5/1943 Wilson 219-1081 X 3,082,710 3/1963 Holland 219-1081 X 3,095,548 6/1963 Manwaring 219-10.75 X

RICHARD M. WOOD, Primary Examiner. L. H. BENDER, Assistant Examiner. 

1. A STRAY FIELD ELECTRODE COMPRISING, IN COMBINATION: FIRST AND SECOND SUBSTANTIALLY CO-PLANAR, SPACED APART ELECTRIALLY CONDUCTIVE PLATE ELECTRODES; AND, A PLURALITY OF ELONGATED, SUBSTANTIALLY V-CROSS-SECTION, POINTED END, ELECTRODE ELEMENTS POSITIONED IN PARALLEL, SPACED-APART RELATION GENERALLY PARALLEL TO AND SPACED APART FROM SAID PLATE ELECTRODES WITH THEIR POINTED ENDS POINTING AWAY FROM SAID PLATE ELECTRODES, SAID ELECTRODE ELEMENTS FORMING TWO SERIES OF ELECTRODE ELEMENTS ALTERNATELY ARRANGED WITH ONE ANOTHER, ONE OF SAID SERIES BEING ELECTRICALLY CONNECTED TO SAID FIRST PLATE ELECTRODE AND THE OTHER OF SAID SERIES BEING ELECTRICALLY CONNECTED TO SAID SECOND PLATE ELECTRODE. 